Heritability.
The proportion of the variance in a particular trait that can be attributed to genetic variations in a given population.

Alternate splicing
Natural usage of different sets of splice junction sequences, to produce more than one protein product from a single gene.

Assortative mating
Non-random mating. 
Could choose similar or dissimilar partners.

Dominance
A dominant allele produces the same phenotype in an individual regardless of whether one or two copies of this allele are present.

D4DR
The gene that codes for the D4 receptor for dopamine. 
Known to have variant forms within the normal population and it has been a candidate for the trait of novelty seeking etc.

Epigenetics
The study of modifications (eg. methylation of DNA) that affect the expression of genes without changing the DNA sequence.

Epistasis
Non-additive interaction between genes at different loci. Ie. cases where the effect of one gene depends on another.
One of the factors that complicate estimates of heritability derived by comparing MZ and DZ twins.

Genotype
Particular combination of alleles that an individual has at a particular locus.

Allele
An alternative form of a particular gene

Heterozygote
Individual who has two different versions of a given gene. (Different alleles at the same loci for the two chromosomes)

Intron
DNA sequence within a gene that is transcribed into mRNA but is spliced out before translation into protein.

Knock-out
An animal (almost always a mouse) from which a particular gene has been removed, as a way of inferring the function of the gene.

Messenger RNA
RNA that carries - from the nucleus of the cell to the ribosomes - a template for synthesis of protein.

Nucleotide
A component of DNA corresponding to the basic element of the genetic code - made up of sugar, base and phosphate.

Polymorphism
a) The presence in a population of more than one form of a given feature or trait
b) The presence of two or more versions (alleles) of a given gene

Promoter region
A region outside the coding part of a gene, but usually nearby and upstream, that controls the amount, time and place of expression of the gene. 
RNA polymerase binds to the promoter region to initiate transcription of the gene.

Single nucleotide polymorphism (SNP)
The presence of alternative nucleotides at a particular position in the sequence of a gene.

Transcription
Construction of mRNA from a section of DNA that serves as a template.

Translation
Conversion of mRNA to protein by assembling amino acids into a chain.

Trait
Any feature or behavioural property in which a given species varies.

Triplet repeat expansion disorder
Class of disorders where a specific sequence of 3 bases is expanded to a length that produces a dysfunctional phenotype.
eg. Huntington disease, Fragile X Syndrome

Whole-genome association study
A research study in which 
a) individuals are scored for some phenotypic characteristic or medical condition and
b) their genomes are probed at 1 million or more SNPs, in order to establish correlations between particular characteristics and particular genetic variants.

Variable number tandem repeat (VNTR)
A short sequence of nucleotides is repeated a number of times.

What is genetic linkage?
Genetic linkage is the tendency of alleles that are located close together on a chromosome to be inherited together during meiosis.